The ending explained - Screenshots - SPOILERS
There are at least three different threads asking questions about the ending, so I figured I'd just create a totally conclusive one. It's definitely the main character in the cage. I'm attaching screenshots.
He apparently didn't die when she stabbed him and now she keeps him in a cage and uses him to release her violent urges so that she doesn't hurt anyone else.
Saying all of this now sounds like, "Yeah, duh, that was obvious," but I didn't think it was obvious when I first viewed the film. I didn't think the man in the cage looked like Seth (plus I thought he was dead). I thought maybe she had just found another random guy to torture.
I really liked the movie, but I feel like they could have made the ending easier to understand. Maybe a little lighter on his makeup or have her say his name. Or have her stab him in the stomach instead of slicing his throat, because it's easier to believe he could have survived that. I just know that I missed it (and based on the comments, I wasn't the only one) and I'm not usually confused by stuff like this.
Screenshots:
http://s135.photobucket.com/user/Lang1725/library/Pet%20Film%20Screenshots
BTW...If you liked this, you should check out the 2005 film Puppy. The same kind of concept, but very different films. That one is more of a drama, not horror. http://www.imdb.com/title/tt0432006/?ref_=fn_al_tt_2